In order to ensure a correct function of sensors, sensors usually are calibrated. For the calibration of sensors there are used calibration media which have certain, predetermined properties. By means of these known properties it is possible to calibrate the sensors, because the known properties of the calibration media have to yield certain, expected measuring signals of the sensors. Thus it is possible to recognize deviations of the sensors, which are caused by e.g. manufacturing, ageing, soiling etc, and to take these into account in measurements. For this purpose, in a calibration mode it is ascertained whether the measuring signals of the sensors deviate from the measuring signals expected due to the calibration medium. The size of the ascertained deviations determines the adjustments required for a correction of the deviations. For this purpose for example correction factors can be determined which are applied to the measuring signals of the sensors upon subsequent measurements in the operating mode in order to compensate for the deviations. Likewise, the intensity of an excitation source, for example an illumination, can be changed upon the correction until the measuring signals of the sensor have the expected values. The changed intensity of the excitation source is maintained and subsequently used for measurements with the sensor, thereby compensating for the occurred deviations.
Special difficulties will arise, if great demands are made on the calibration of sensors, because by means of the sensors there are carried out sensible measurements, e.g. the recognition of documents of value, in the following referred to as bank notes, whose type (currency, denomination), authenticity, state (soiling, damage) etc is to be ascertained. Such cases require a very precise calibration of the sensors, since misjudgments due to wrong measuring signals of the sensors must be absolutely avoided, so that besides a white balance, necessarily, a color balance must also be carried out in order to certainly avoid misjudgments. In this connection, the use of high-quality calibration media having standardized references, so-called measuring standards, has also turned out to be problematic for various reasons.
From WO 2006/025846 A1 there is known a self-calibrating optical system which uses a high-quality calibration medium having a standardized white reference which is integrated in a sensor housing of the optical system. The calibration medium having the standardized white reference is pivoted into the optical path of the sensor during the calibration mode by means of a mechanism within the sensor housing. Due to the defined optical properties of the standardized white reference a self-calibration of the sensor is possible at any time. In the operating mode the calibration medium is pivoted out of the optical path of the sensor by means of the mechanism, in order to permit measurements of e.g. pharmaceutical products. There is additionally proposed to use a further calibration medium which has certain spectral properties. For this further calibration medium there is also provided a further mechanism to permit the further calibration medium to be pivoted into and out of the optical path of the sensor within the sensor housing.
The self-calibrating optical system known from WO 2006/025846 A1, however, has the disadvantage that for each calibration medium to be used a separate mechanism must be used to permit the calibration media to be successively pivoted into and out of the optical path of the sensor, so that the latter can be calibrated by means of the calibration media having different properties, for example a white and color balance can be performed.
A further problem arises from the fact that high-quality calibration media having standardized references have to be used so as to penult the desired self-calibration at any time. The use of the standardized references on the one hand has the disadvantage that such standardized references have to be employed in each optical system to be calibrated, but such standardized references for the calibration are expensive. This results from the necessity to exactly measure the standardized references, since for the self-calibration it has to be ensured that the references exactly have the desired properties. On the other hand, despite the relatively protected accommodation of the standardized reference it may come to changes of the standardized reference e.g. due to ageing. In this case a reliable self-calibration of the sensor is no longer possible.
Furthermore, due to the accommodation of the calibration media having the standardized references within the sensor housing and the pivoting into the optical path of the sensor to be calibrated within the sensor housing there is always given a deviation from the actual measuring place of the sensor which is located outside the sensor housing. This problem is further aggravated, when the sensors or an associated illumination are to detect larger line-shaped or areal regions, so that they are built up, for example, as a line camera. Such sensors have a multiplicity of elements which are arranged side by side so as to form e.g. the line-shaped sensor or its illumination with a required length. Normally, such sensors or illuminations additionally have optical imaging systems. In such cases on the one hand it is desirable to perform a calibration for all the elements forming the sensor, on the other hand upon the known calibration there occurs blur, because the calibration medium does not lie in the focus area of the sensor within which the measurement of measuring objects, e.g. bank notes, is effected in the operating mode.